Benjamin Reuter

Evaluation of slope stability with respect to snowpack spatial variability

The evaluation of avalanche release conditions constitutes a great challenge for risk assessment in mountainous areas. The spatial variability of snowpack properties has an important impact on snow slope stability and thus on avalanche formation, since it strongly influences failure initiation and crack propagation in weak snow layers. Hence, the determination of the link between these spatial variations and slope stability is very important, in particular, for avalanche public forecasting. In this study, a statistical-mechanical model of the slab-weak layer (WL) system relying on stochastic finite element simulations is used to investigate snowpack stability and avalanche release probability for spontaneously releasing avalanches. This model accounts, in particular, for the spatial variations of WL shear strength and stress redistribution by elasticity of the slab. We show how avalanche release probability can be computed from release depth distributions, which allows us to study the influence of WL spatial variations and slab properties on slope stability. The importance of smoothing effects by slab elasticity is verified and the crucial impact of spatial variation characteristics on the so-called knock-down effect on slope stability is revisited using this model. Finally, critical length values are computed from the simulations as a function of the various model parameters and are compared to field data obtained with propagation saw tests.

Snow instability patterns at the scale of a small basin

Spatial and temporal variations are inherent characteristics of the alpine snow cover. Spatial heterogeneity is supposed to control the avalanche release probability by either hindering extensive crack propagation or facilitating localized failure initiation. Though a link between spatial snow instability variations and meteorological forcing is anticipated, it has not been quantitatively shown yet. We recorded snow penetration resistance profiles with the snow micropenetrometer at an alpine field site during five field campaigns in Eastern Switzerland. For each of about 150 vertical profiles sampled per day a failure initiation criterion and the critical crack length were calculated. For both criteria we analyzed their spatial structure and predicted snow instability in the basin by external drift kriging. The regression models were based on terrain and snow depth data. Slope aspect was the most prominent driver, but significant covariates varied depending on the situation. Residual autocorrelation ranges were shorter than the ones of the terrain suggesting external influences possibly due to meteorological forcing. To explore the causes of the instability patterns we repeated the geostatistical analysis with snow cover model output as covariate data for one case. The observed variations of snow instability were related to variations in slab layer properties which were caused by preferential deposition of precipitation and differences in energy input at the snow surface during the formation period of the slab layers. Our results suggest that 3-D snow cover modeling allows reproducing some of the snow property variations related to snow instability, but in future work all relevant micrometeorological spatial interactions should be considered.

Effects of snow properties on humans breathing into an artificial air pocket - an experimental field study

Breathing under snow, e.g. while buried by a snow avalanche, is possible in the presence of an air pocket, but limited in time as hypoxia and hypercapnia rapidly develop. Snow properties influence levels of hypoxia and hypercapnia, but their effects on ventilation and oxygenation in humans are not fully elucidated yet. We report that in healthy subjects breathing into snow with an artificial air pocket, snow density had a direct influence on ventilation, oxygenation and exhaled CO2. We found that a rapid decline in O2 and increase in CO2 were mainly associated with higher snow densities and led to premature interruption due to critical hypoxia (SpO2 ≤ 75%). However, subjects in the low snow density group demonstrated a higher frequency of test interruptions than expected, due to clinical symptoms related to a rapid CO2 accumulation in the air pocket. Snow properties determine the oxygen support by diffusion from the surrounding snow and the clearance of CO2 by diffusion and absorption. Thus, snow properties are co-responsible for survival during avalanche burial.

Describing Snow Instability by Failure Initiation, Crack Propagation, and Slab Tensile Support

Snow instability is a generic term describing the propensity of a snow slope to avalanche. In need of a concise mechanics-based concept we suggest a framework based on failure initiation, crack propagation, and slab tensile support. Following these three steps we modeled three metrics from mechanical data, which we derived from snow micropenetrometer signals. Verifying the metrics with field measurements confirmed that slab thickness and weak layer strength typically influence failure initiation, elastic modulus and weak layer fracture energy largely control crack propagation, and slab thickness and tensile strength provide the required tensile support. For all three metrics, considering slab layering was essential. Validation with signs of instability showed that the most accurate model includes all three steps – suggesting that snow instability can be described by failure initiation, crack propagation, and slab tensile support. Further validation is needed to assess the framework’s potential for operational use. Plain Language Summary Snow avalanches threaten people and infrastructure in all snow-covered mountain regions. Avalanche mitigation includes avoiding avalanches in space and time. The latter requires avalanche forecasting, that is, predicting snow instability or the probability of a snowpack to avalanche. As properties of the mountain snowpack vary spatially and evolve with time, modeling approaches are better suited than point observations to assess snow instability. However, for simulating snow instability in snow cover models quantitative measures are needed, preferably reflecting the processes leading to slab avalanche release. Dry snow slab avalanche release can be interpreted as a sequence of fractures, including failure initiation and crack propagation. We follow these steps and complement our recently developed metrics describing failure initiation and crack propagation with a criterion for the tensile support of the slab. We identify the snow properties that are most relevant to drive the three processes and validate the metrics with independent field observations. The validation confirmed that snow instability is best interpreted as a combination of failure initiation, crack propagation, and slab tensile support. Our new model to describe snow instability on mountain slopes is now ready to be assessed in operational use for avalanche forecasting.